1. Technical Field of the Invention
The present invention relates to combustion pressure sensors for use in internal combustion engines and, more particularly, to a combustion pressure sensor for detecting a combustion pressure in an internal combustion engine such as an automotive diesel engine combined with a glow plug function for preheating an air-fuel mixture.
2. Description of the Related Art
In related art, an attempt has heretofore been made to provide a combustion pressure sensor combined with a glow plug function for a diesel engine, with one exemplary structure being disclosed in Japanese Patent Application Publication No. 2005-90954. With such a structure of the combustion pressure sensor, a glow plug body includes an upper portion for connection to an engine head, and a lower portion exposed to a combustion chamber of a diesel engine. The glow plug body is internally provided with a heating rod, with a film being sandwiched between a lower end of the upper body and the upper area of the lower portion. The film is strongly fixed to between the upper body and the lower portion by laser welding to transfer a heat, developed by the heating rod, to the glow plug body. In addition, the film guarantees a ground connection required for the glow function of a pressure glow plug, while maintaining a gastight sealing effect for the connecting side of the glow plug. In addition, the upper body of the glow plug body has the connecting side whose end face carries thereon a pressure sensor. The pressure sensor is interposed between the glow plug body and a connecting tube under the application of a preliminary pressure.
Further, the engine head is formed with a female-thread and the upper body of the glow plug body has an outer periphery formed with a male-thread. When mounting the combustion pressure sensor onto the engine head, the male-thread of the glow plug body is screwed into the female-thread of the engine head in a gastight sealing effect to bock a leakage of the combustion pressure through a clearance between the outer periphery of the glow plug body and an inner wall of a mounting bore of the engine head. Further, a distal end of the lower portion of the glow plug body is brought into tight contact with a contact surface of the mounting bore of the engine head through the use of a metallic seal member.
With the combustion pressure sensor mounted on the engine head, a combustion pressure arising in a combustion chamber of the diesel engine acts on the heating rod to cause the same to move in an axial direction. This causes the film to move a contact tube toward the upper portion. Such axial movement is transferred to a tension element, thereby releasing the pressure sensor from the preliminary pressure exerted thereto in a preceding stage. Upon releasing of such a preliminary pressure from the pressure sensor, the pressure sensor can detect the combustion pressure to generate an output, which in turn is applied to an electronic circuitry.
With the combustion pressure sensor of the structure set for above, however, when mounting the combustion pressure sensor onto the internal combustion engine, a contraction occurs on the glow plug body in shape along an axial direction thereof. In this moment, such a contraction of the glow plug body causes the heating rod, held in a bonding state with the glow plug body via the film having a rigidity, to move relative to the glow plug body in the axial direction thereof toward the pressure sensor. Upon the occurrence of such an axial movement to transfer a load to the pressure sensor, the pressure sensor is subjected to an undesired load. This results in a deviation in the preliminary pressure exerted to the pressure sensor. This results in a fear of the initially exerted preliminary pressure being deviated, causing an erroneous output different from a correct output to be inevitably generated.